1. Field of the Invention
The present invention relates to a multi-antenna system. More particularly, the present invention relates to an apparatus and method for removing interference in a transmitting end of the multi-antenna system.
2. Description of the Related Art
A multi-user multi-antenna system has conventionally employed either a Zero-Forcing (ZF) scheme or a Minimum Mean Square Error (MMSE) scheme. In the ZF scheme, a signal transmitted from a transmitting end (i.e., Base Station (BS)) is multiplied by an inverse of a channel matrix so as to reduce interference caused by a different Mobile Station (MS) or a different antenna. In the MMSE scheme, signal transmission is achieved in consideration of a channel noise variation.
The ZF scheme and the MMSE scheme have advantages in that a transmitting end can be easily implemented, and an error rate is not significantly increased even when the amount of channel feedback information transmitted from MSs is not sufficient. In particular, several schemes are actively being discussed in many standardization organizations such as the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), wherein such schemes employ a structure in which, instead of feeding back entire channel information, each MS selects a suitable vector from a pre-defined codebook and feeds back a codebook index and Channel Quality Information (CQI), and a transmitting end then utilizes channel information received from each MS and thus performs a scheduling operation so that sum capacity can be maximized.
The ZF scheme and the MMSE scheme are based on linear pre-coding. On the other hand, some examples of schemes based on non-linear pre-coding include a Tomlinson-Harashima Precoding (THP) scheme in which Dirty Paper Coding (DPC) is applied to a one-dimensional vector and a Vector Perturbation (VP) scheme in which the DPC is applied to an nth dimensional vector. In such a non-linear pre-coding scheme, a receiving end (i.e., MS) sends accurate Channel State Information (CSI) or its equivalent to a transmitting end, and the transmitting end allows a transmission signal to be subject to a modulo operation so that a positive integer value is added to or subtracted from the transmission signal. Even when the receiving end does not know the positive integer value, the receiving end can estimate the signal through the same modulo operation as applied at the transmitting end. Accordingly, the transmitting end can optimize both a channel and a transmission signal. Hence, the non-linear pre-coding scheme has been researched as a promising technology in a Time Division Duplex (TDD) nomadic environment where feedback is frequently made to the transmitting end.
Meanwhile, the ZF scheme and the MMSE scheme have demerits as follows: performance deterioration and transmission power loss are inevitable; transmission power has to be constant for each MS or each antenna; each MS has to use only one antenna; or, in particular, discrepancy between sum capacity and ideal capacity becomes significant as Signal-to-Noise Ratio (SNR) increases.
Moreover, the DPC-based non-linear scheme has demerits as follows: a data transfer rate has to be constant for each MS; and each MS has to use only one antenna. Therefore, disadvantageously, Quality of Service (QoS) for each MS cannot be properly ensured.
Accordingly, there is a demand for a method in which performance can be maximized by optimizing a data transfer rate and transmission power for each MS in a multi-antenna system.